The 7 Sexes: Biology of Sex Determination

The 7 Sexes: Biology of Sex Determination

by Elof Axel Carlson, Becky Phillips

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Few of us know much about the biology of sex determination, but what could be more interesting than to discover how we are shaped into males and females? In this book, Elof Carlson tells the incredible story of the difficult quest to understand how the body forms girls and boys. Carlson’s history takes us from antiquity to the present day to detail how each

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Few of us know much about the biology of sex determination, but what could be more interesting than to discover how we are shaped into males and females? In this book, Elof Carlson tells the incredible story of the difficult quest to understand how the body forms girls and boys. Carlson’s history takes us from antiquity to the present day to detail how each component of human reproduction and sexuality was identified and studied, how this knowledge enlarged our understanding of sex determination, and how it was employed to interpret such little understood aspects of human biology as the origin of intersex births.

Editorial Reviews

Johan Verhulst

"Carlson does an excellent job presenting the researchers who, over time, worked out the details of sex determination and differentiation. Highly informative and instructive." —Johan Verhulst, MD, Kinsey Institute

Barbara Baumgartner

"Carlson reveals the various scientific advances, in both methods and understanding, that allowed researchers to identify anatomical structures, gametes, sex hormones, sex chromosomes, and ultimately, a firm comprehension of the genetics of sex.... This scientific story of discovery is quite fascinating." —Barbara Baumgartner, Washington University, St. Louis

From the Publisher
"Carlson reveals the various scientific advances, in both methods and understanding, that allowed researchers to identify anatomical structures, gametes, sex hormones, sex chromosomes, and ultimately, a firm comprehension of the genetics of sex.... This scientific story of discovery is quite fascinating." —Barbara Baumgartner, Washington University, St. Louis

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The 7 Sexes

Biology of Sex Determination

By Elof Axel Carlson

Indiana University Press

Copyright © 2013 Elof Axel Carlson
All rights reserved.
ISBN: 978-0-253-00654-7



With rare exceptions, animals consist of sexually reproducing populations that are roughly half male and half female—at least that is a human perspective that is applied to other mammals, and generalized to all other animals. An observant individual will notice roaches mating rear end to rear end or horseshoe crabs on the beach in springtime mating with the male mounted on a female, reinforcing the idea that the image of human intercourse can be generalized. I can observe fruit flies mating in the same way without use of a microscope, and I can even tell which is male and which is female if I am looking at a solitary fruit fly resting on my finger.

But that idea of universality is undermined if I observe copulating earthworms, which seem to be engaged in some sort of symmetrical mutual engagement. The ambiguity of the earthworm's hermaphroditism is also present in most flowering plants. Students learn that pollen bearing stamens are present in the same flower with female components—assigned scholarly names like stigma, style, and ovary—but that is also not universal.

Until the invention of microscopes in the 1660s, the world of the very small organisms—or parts of organisms, like cells—was closed off to human observation; almost all early ideas of sex determination are rooted in what could be seen with the unassisted eye. The "two by two" image of sexuality is reinforced in the story of Noah's ark, but the story of sex determination in the Bible is puzzling to a reflective reader. Adam, clearly a functional male at creation, is given a companion out of his own rib and she is called Eve, but the special creation of the female is necessary only in the human species. Water, air, and land animals are created in Genesis equally as male and female (one assumes) in an indeterminate number with instructions to be fruitful and multiply; there is no "second sex" creation for the rest of life.

This book is a history of our ideas about sex determination—from ancient myths to present day molecular insights. I got interested in the history of human sexuality after teaching a course on the biology of human reproduction at Stony Brook University. Sometime around 1990, Ruth Cowan asked me if I knew anyone who could teach such a course for an undergraduate program she was developing. I volunteered to teach the course myself and spent a summer at the medical school library preparing the lectures and a text (unpublished) for it. The course was well received and I continued to teach it until I retired in 2001.

When there was no science, as we know it, speculation associated with religious writings prevailed on the determination of sex: Judaism, Christianity, and Islam drew their interpretations from the book of Genesis. Other religious traditions abound, which folklore scholars have studied using appropriate motifs of gender, sexual reproduction, twinning, hermaphroditism, and other variations. In the western tradition, the first scholarly attempts at studying sex determination are associated with Greek philosophers, but their insights are not very helpful because the tools of science were absent. Humans are adept at framing stories that explain complex reality as best as they can, but observation alone provides limited information.

The turning point in the history of sex determination arose with the introduction of the microscope when, for the first time, microscopic anatomy was revealed. It then took almost two centuries after cells were first named for the cell theory to emerge. Almost all of our present day knowledge of the biology of sex determination is relatively recent, primarily worked out in the twentieth century. For the duration of written history prior to the twentieth century, sex determination was embedded in religious traditions, taboos, and moral transgressions. Severe penalties could be imposed if those violating these norms were identified and tried by religious or secular courts.

Each component of human sexuality has its own history. The story of sperm, eggs, gonads, external genitals, internal genitals, sex in different stages of the life cycle, pregnancy, twinning, hormonal regulation, fertilization, alternation of haploid and diploid cellular states, the role of meiosis, sex chromosomes, and many other features of the complex events and developmental anatomy of reproduction was worked out and by the 1960s a fairly modern understanding was available. All that was lacking was a molecular interpretation of the way sex determination works, and that too yielded to studies in the last third of the twentieth century.

In addition to the normal sequence of events leading to functional males and functional females copulating as heterosexual couples, a number of variations existed. Some involved rare ambiguities of genitals called, at the time, hermaphroditism, chimerism, or pseudohermaphroditism. Some involved equally rare disturbances of sexual development associated with sex chromosome aneuploidy. Some involved possible genetic or gestational conditions that led to changes in sexual orientation including homosexuality, which is especially of interest to society because of its wide prevalence.

Independent of these primarily biological changes, which could be worked out by scientists, there were changes in society's views on gender roles. The rise of the women's movement in the nineteenth century led to the achievement of social equality in the late twentieth century in most industrialized nations, including the United States.

Because human sexuality was so deeply connected to religious moral teachings, the scientific findings about human sexuality have been controversial; many are rejected either in courts of law, legislatures, or by public opinion. This is not surprising because the existence of a scientific understanding of human sex determination is almost entirely a consequence of findings that are less than a century old.

The study of comparative sex determination in animals, plants, and microbial organisms produces insights into the way genetic transmission takes place across the phyla. It also gives insights into the evolutionary history of sex determination. These insights do not have as much social impact as discoveries associated with human sex determination have. If we think about some essential insights into the human body and compare them to insights about sex determination, we begin to see why public understanding is so meager about our own sexuality. Gross anatomy, worked out by Andreas Vesalius (1514–1564), has been around for more than 400 years. Today, no one feels threatened by the idea that their bodies are composed of organs, bones, muscles, nerves, a vascular system, and other components they share with most animals. Similarly, the cell theory is almost 200 years old and we accept the reality that cells form tissues and that these tissues form our organs. Descriptive embryology is also about 200 years old and we are not threatened that we can follow a human life from a fertilized egg through blastocyst formation, implantation, embryonic tissue formation, and a cascade of developmental events that result in our organ formation and body symmetry.

Recency of discovery is only one factor in the lack of public understanding of human sex determination. The taboos associated with sexual knowledge are still strong in society. Because of this, it is poorly taught in K–12 education. Yet it will concern almost every individual who emerges out of childhood innocence and who is thrust into sexual life during the teen years. Morality tells us how to behave and it is proper that parents instruct their children. But if parents who are ignorant of the biology and chemistry of sex determination instruct their children, a lot of ignorance will be passed on.

We like to think, as scientists, that the more we know, the more options we have in a democratic society to explore what is best for ourselves, our families, and society. That is not borne out with public or religious sex education, which provides little of what is found in this book. If "the truth shall make you free," why is that knowledge, hard won by science, shielded from most of humanity? That, of course, is not a question science asks. As scientists we provide information about the material universe we explore, describe, and interpret. It is up to society to use that knowledge, but it is frustrating nevertheless when it is ignored or rejected because it is inherently controversial to know our own sex determination.

Those involved in the feminist movements, and those who study gender and its shifting social and philosophic place in history, are using what I would call a top- down approach to sexuality that is very similar to experimental psychology. But biologists use a bottom-up approach: they want to understand how life works by following it from molecules, genes, cells, organs, and organisms through to evolution and society. The later chapters of this book try to relate these two approaches.

What you will not find in this book is a very popular approach called sociobiology, evolutionary psychology, or claims of an innate basis for most of that human activity that we call sexual behavior or gender identification. I discuss my guarded view of these approaches in the last two chapters.

What I hope I have achieved is a more detailed account of the history of the biology of sex determination and the historical changes in our views of human sexual differences in behavior. We have learned a substantial amount of knowledge about the processes of sex determination and differentiation and the underlying genetic and molecular events involved in those events. The same cannot yet be said for the genetic and molecular processes (if any) for human sexual and gender behavior.


Wild Guesses in an Era of Scientific Ignorance

Almost all of the topics taught in K–12 or undergraduate introductory science courses come from work published in the last two centuries. Before the nineteenth century, very little of the chemistry, biology, geology, astronomy, or physics (other than Newtonian) that is covered in a twenty-first century class had been discovered. Almost all of a medical school curriculum, with the exception of gross anatomy, is a product of work done in the past two centuries. However, humanity centuries ago had the same curiosity about life and the universe as those born today. One of the universally recognized experiences of all people born is that roughly half are males and about half are females. When it comes to classifying who is a male and who is a female at birth, almost every adult in the world will use the external genitals. In a male there are a penis and a scrotum containing two testes. In a female there is a vaginal passageway surrounded by labia and a clitoris. Except for rare occasions we do not see the genitalia of our fellow adult human beings. Usually we classify a person as male or female by characteristics such as body shape, the presence or absence of hair on the face, the length, distribution, and style of hair from the cranial part of the head, the bony structure of the limbs and face, the deepness or higher pitch of voice, the presence or absence of enlarged breasts, and the presence or absence of an "Adam's apple." To that, in most cultures, we add the clothing that people wear and the behavior we assign to males or females out of tradition or experience.

For most people humanity falls into two categories: half are male and half are female. There may be cultural reasons why a male or a female baby is preferred, but even in cultures in which there is no such cultural bias, prospective parents and family members are curious about the sex of the child before it is born, and virtually every period of history has left cultural evidence behind that it believed it could either predict the sex of a baby or actually manipulate it to a desired sex before it is born.

The oldest document recording such beliefs is the Berlin Papyrus, which was written about 1400–1600 bce (i.e., some 3500 years ago). The author claims that if two pots are set up with a mixture of sand and dates, and if urine is added from a pregnant woman to both pots, and if seeds of barley are placed in one pot and seeds of wheat in the other, then the child will be a boy if the wheat sprouts first and a girl if the barley sprouts first. Some current folk beliefs have not varied since antiquity. Many in Appalachian regions of the United States still believe that a male who loses a testicle will be capable of producing only one sex of children. Some folk beliefs claimed that males come from the right testis and females from the left testis. A variant of that is that males come from the embryo implanted on the right side of the uterus, females from embryos implanted on the left side. Further variations on these themes take breast size into account. Some believe that if the right breast swells larger than the left breast the baby will be a boy. There is no reason given why this should be so.

A more rational argument claims that young or very old males are more likely to have daughters, while males in their prime are likely to have sons. This reflects a belief that males are more active or aggressive than females, and assumes the fate of the sperm is determined by the behavior of the male. Similar logic leads to the belief in many cultures that women who are pregnant experience more active movement if it is a boy than if it is a girl. A similar active/inactive principle is at work in the folk belief that a pregnant woman who bathes in colder water (or who is exposed to cold water by rain) is more likely to have a daughter. Here hot/cold are related to the male as hotter (sweats more) than the female.

Some cultures used astronomical signs. In Assyrian culture, a halo around the moon indicated a time to copulate if the couple desired a son. Among ancient Hindus if a woman had excess menstrual flows she was prone to producing daughters. Some folk myths were based on diet. For example, eating a rooster's testes after intercourse would assure birth of a son, as would eating more meat, especially red meat. More recent folk myths in North America include a belief that a large rounded pregnancy was a sign of the birth of a boy. Also, if morning sickness is frequent and severe, this must be a sign of carrying a boy. Almost every folk belief put to the test has been shown to be no more helpful than flipping a coin.

For farmers, a similar effort was made to predict or control the sex of cattle, sheep, chickens, or other animals of interest. Some farmers believed that the direction of the wind played an influence, sheep copulating in a south wind produce females but those facing a north wind produced males. Some also believed that chicken eggs with a pointed end are more likely to yield hens and those with blunt ends will produce roosters.


If people in the past felt it important to know why they were going to have a boy or a girl, they were equally curious about having twins. Twins occur about once in 80 births in most parts of the world. There are two types of twins. Non- identical or dizygotic twins are produced when two eggs which happened to ovulate about the same time in the ovary and which are implanted separately in the uterus are fertilized. One fourth will be male/male; one fourth will be female/female; and half will be a boy and a girl. A second category of twins involves same-sex twins who look alike. They are called identical twins or monozygotic twins. Of course, the idea of a zygote is a late nineteenth century finding; before there were microscopes, there was no conception of a zygote or fertilized egg. It was as much of a puzzle why twins occurred as why identical twins always had the same sex. In African folklore, the Yoruba of Nigeria believed that twins were a consequence of two acts of intercourse, one with a human male and the other with a deity. This gave the twins a dual status: one was a mortal and the other was a demigod (but no one knew which was which). For the Yoruba, twins were honored and raised in the royal household or given special favors in the communities where they were born, often with double rations of the products they bought. This practice, over many generations, may have selected for genes associated with multiple ovulation because the frequency of dizygotic twins among the Yoruba is among the highest in the world—about one in every 25 pregnancies. In the United States, the frequency of dizygotic twins was about one in 70 until the 1970s, when it began to climb to one in 50 because of the introduction of assisted fertility, especially in vitro fertilization, and because women were having their children at an older maternal age. Environmental as well as genetic factors influence the incidence of dizygotic twinning. In contrast, monozygotic twins are relative rare, about one in 400 pregnancies and that is a frequency not under genetic control. In Japan, because folk tradition looked unkindly on departures from normalcy, twins and children with birth defects were put to death or abandoned. This may have led to the relative scarcity of dizygotic twinning in Japan, where they are about as frequent as identical twins.


Excerpted from The 7 Sexes by Elof Axel Carlson. Copyright © 2013 Elof Axel Carlson. Excerpted by permission of Indiana University Press.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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